Many bioactive agents including pharmaceuticals, nutrients, vitamins and so forth have a “functional window”. That is to say that there is a range of concentrations over which these agents can be observed to provide some biological effect. Where the concentration in the appropriate part of the body (e.g. locally or as demonstrated by serum concentration) falls below a certain level, no beneficial effect can be attributed to the agent. Similarly, there is generally an upper concentration level above which no further benefit is derived by increasing the concentration. In some cases increasing the concentration above a particular level results in undesirable or even dangerous effects.
Some bioactive agents have a long biological half-life and/or a wide functional window and thus may be administered occasionally, maintaining a functional biological concentration over a substantial period of time (e.g. 6 hours to several days). In other cases the rate of clearance is high and/or the functional window is narrow and thus to maintain a biological concentration within this window regular (or even continuous) doses of a small amount are required. This can be particularly difficult where non-oral routes of administration (e.g. parenteral administration) are desirable or necessary, since self-administration may be difficult and thus cause inconvenience and/or poor compliance. In such cases it would be advantageous for a single administration to provide active agent at a therapeutic level over the whole period during which activity is needed.
Some patients undergoing treatment will typically require a therapeutic dose to be maintained for a considerable period and/or ongoing treatment for many months or years. Thus a depot system allowing loading and controlled release of a larger dose over a longer period would offer a considerable advantage over conventional delivery systems.
Certain of the formulations of the present invention generate a non-lamellar liquid crystalline phase following administration. The use of non-lamellar phase structures (such as liquid crystalline phases) in the delivery of bioactive agents is now relatively well established. A most effective lipid depot system is described in WO2005/117830, and a highly preferred lipid depot is described in that document. However, there remains scope for achieving depot formulations having improved performance in several respects.
Lipid controlled-release delivery systems have been developed with active agents including GLP-1 (WO2006/131730), somatostatin analogues (WO2006/075124), LHRH analogues (WO2006/075125), as well as non-peptides such as buprenorphine (WO2014/016428). Lipid systems are also of value in treatment in their own right and need not include active agents. For example, the FDA approved oral liquid Episil® alleviates the pain caused by oral mucositis and other inflammatory conditions of the mouth by forming a lipid barrier in the oral cavity, but does not require any active agent.
A particularly versatile combination of lipids is glycerol dioleate (GDO) and phosphatidyl choline (PC). However, sustained released formulations can be produced with a wide variety of other lipid components including tocopherol (WO2006/075123), derivatives of sorbitol (WO2016/102683), triglycerides (WO2016/066655), and a variety of phospholipid components including phosphatidyl ethanolamines (WO2013/083459 and WO2013/083460).
Both the lipid components, particularly unsaturated lipids, and any active agent contained in the pre-formulation or sustained release composition are susceptible to oxidation, either during storage or in vivo. It is desirable to decrease the extent of oxidation since oxidation processes may reduce the content of active agent and/or contribute to the formation of unwanted decomposition products. This in turn reduces the shelf life of a product.
One particular factor contributing to oxidation in lipid compositions is the presence of trace amounts of metal ions, particularly transition metals such as iron (Fe). Even when the lipid components are of high purity grade it is often difficult to entirely remove traces of such ions. It is thought that equipment used for the manufacture of lipid formulations commonly includes stainless steel which can leach small amounts of metal ions (particularly Fe) into the mixture. It is therefore common to include an antioxidant in lipid formulations. These generally function by chelating any metal ions, thereby hindering their participation in oxidation processes.
It is a prerequisite that any antioxidant must be soluble in the lipid pre-formulation. It is described in WO2012/160213 that a carefully controlled amount of water can be included in lipid pre-formulations without causing a phase change into a liquid crystalline phase. In pre-formulations containing an appreciable aqueous content, it may be possible to include an effective amount of a water-soluble antioxidant such as ascorbic acid, inorganic salts of metal chelators, such as ethylenediaminetetraacetic acid (EDTA) (e.g. sodium or calcium salts) and citric acid. However, for certain active agents it may be necessary to avoid prolonged exposure to water during storage (e.g. because the active agent is moisture sensitive), or a more desirable release profile may be obtained without the inclusion of water in the pre-formulation. The present inventors have established that certain somatostatin receptor agonists are less stable in formulations containing water. The avoidance of water may also reduce the amount of trace metals which may be present, since metal ions are generally more soluble in water than in an organic solvent or lipid environment. In lipid formulations having a low water content it is not possible to use conventional water-soluble antioxidants since these may not have the requisite solubility in a substantially water-free lipid environment. It would therefore be advantageous to provide an antioxidant which is soluble in a substantially water-free lipid environment and which limits or prevents the oxidative degradation of the lipid components of the pre-formulation, and any active agent contained within. This is particularly the case for metal chelating agents such as EDTA where the standard inorganic salts (sodium or calcium) are non-soluble or have negligible solubility in non-aqueous environments (e.g. lipid matrices).
WO2010/020794 describes thiolated antioxidants as offering particular advantages in lipid systems and suggests that these are also suitable in non-aqueous lipid systems. However, for certain end uses the presence of a thiolated antioxidant may not be acceptable. This particularly applies, for example, to peptides or proteins having thiolated groups or disulphide bridges. WO2010/020794 also mentions the possibility of including EDTA or the sodium, disodium and calcium disodium salts of EDTA as chelating agent although this is not an option which is exemplified. The present inventors have established that EDTA or the common salts thereof are not soluble to any appreciable extent in the types of lipid formulations described in WO2010/020794, i.e. those based on GDO, SPC and an organic solvent such as ethanol.
It has now surprisingly been established that effective amounts of alkylammonium salts of EDTA can be dissolved in a non-aqueous lipid environment, and that the resulting pre-formulations are highly resistant to oxidative decomposition during storage. Furthermore, although alkylammonium EDTA salts are believed to have an effect on decreasing the decomposition by the expected mechanism of sequestering metal ions, the present invention may in some embodiments improve oxidation resistance above the level that can be accounted for solely by this mechanism.
The inventors have established that the inclusion of alkylammonium EDTA salts can prevent, or substantially decrease the rate of, oxidation of a wide variety of lipid components and/or active agents contained therein. The inventors have found that the inclusion of alkylammonium EDTA can substantially reduce the loss of assay of somatostatin receptor agonists in drug samples tested in stability studies and thus increases shelf-life of the drug product. EDTA salts have the advantage that they are inexpensive, easily produced with a wide variety of countercations, and are generally regarded as safe (and are widely used e.g. in pharmaceutical applications).
The stabilizing and shelf-life extending effect of alkylammonium EDTA as found by the inventors may be not only related to the prevention or reduction of oxidation reactions but may be also related to the prevention or reduction of other chemical degradation reactions, e.g. hydrolysis, acylation, deamidation.